1. Field of the Invention
The invention relates to a process for the preparation of acylbenzenes by reaction of benzene with carboxylic acids on acid catalysts.
Acylbenzenes are intermediates which can be widely employed for the synthesis of active compounds in the field of medicaments and plant protection. Benzophenone is used, in addition, in relatively large amounts as odour-fixing agent for perfumes and soaps.
2. Description of the Related Art
Acylbenzenes are generally prepared by acylation of benzene with acyl chlorides using Friedel-Crafts catalysts such as aluminium chloride, zinc chloride, iron chloride and similar metal halides known to those skilled in the art. These Friedel-Crafts catalysts are generally required in stoichiometric amounts. During the workup of acylation mixtures of this type the Friedel-Crafts catalysts are destroyed by hydrolysis and produce relatively large amounts of hydrochloric acid in the off-gas or in the effluent. This hydrochloric acid, which has to be disposed of, originates both from the catalyst and also from the acyl chloride employed for the acylation. In addition to this disposal as a considerable environmental problem, the corrosion problem, which is also caused by the hydrochloric acid, must also be solved. The aim is, therefore, to find a process which is both environmentally friendly and also inexpensive with respect to the disadvantages indicated.
A first attempt to achieve this aim is described in German Offenlegungsschrift 26 16 583, in which it is proposed to prepare aryl ketones by reaction of non-hydroxylated aromatic compounds with a carboxylic acid or a carboxylic acid anhydride in the vapour phase at 250.degree. to 500.degree. C. in the presence of an acid silica/alumina catalyst having a surface area of at least 50 m.sup.3 /g. However, with this process using amorphous or crystalline silica/alumina catalysts undesired by-products of very different structures are formed, for example in the case of the reaction of benzene with benzoic acid, biphenyl, diphenylmethane and alkylated benzenes are also formed in addition to the desired benzophenone.
A particularly serious disadvantage of this process is the short life of the catalysts; thus, on comparison of Examples 2 and 3 of German Offenlegungsschrift 26 16 583, the amount of the desired benzophenone drops considerably if the reaction time is increased to 24 h.